Purified water may be obtained from impure water by osmotic separation processes. Osmosis is the movement of a solvent across a semipermeable membrane towards a higher concentration of solute.
Osmosis may be used directly to achieve separation of water from a solution containing unwanted solutes such as impurities. Forward osmosis (FO) is an osmotic process that, like reverse osmosis (RO), uses a semi-permeable membrane to effect separation of water from dissolved solutes. The driving force in FO is an osmotic pressure gradient, such that a “draw” solution of high concentration relative to that of the feed solution, is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feed water from its solutes. In contrast, the reverse osmosis process uses hydraulic pressure as the driving force for separation, which serves to counteract the osmotic pressure gradient that would otherwise favor water flux from the permeate to the feed. In FO the diluted draw solution may be used as such or sent to a secondary separation process for the removal of the draw solute.
Forward osmosis is an area of ongoing research, focusing on applications in desalination, water purification, water treatment and the like. One area of research in FO and regeneration of draw solutions involves direct removal of draw solutes by means of e.g. a magnetic field. Small nanoscale magnetic particles i.e. magnetic nanoparticles (MNP) are suspended in solution creating osmotic pressure sufficient for the separation of water from a dilute feed. Once the draw solution containing these particles has been diluted by the FO water flux, they may be separated from that solution by use of a magnet.
In order to increase the dispersibility of MNPs the surfaces thereof may be modified by functionalization. Current hydrophilic MNPs for use in forward osmosis are often prepared using a thermal decomposition method which requires elevated temperature. Moreover, the available methods typically involve utilization of high-boiling point organic solvents.
For example, WO2011/099941 discloses a hydrophilic magnetic nanoparticle including a magnetic core composed of MFe2O4 or Fe2O3 and a plurality of hydrophilic polymers covalently bound to the magnetic core. The preferred hydrophilic polymers are poly(ethylene glycol) diacids, polyacrylic acids, poly(styrene)-block-poly(acrylic acid)s, poly(acrylic acid-co-maleic acid)s, poly(acrylamide-co-acrylic acid)s and poly(vinyl acetate-co-crotonic acid)s. The hydrophilic magnetic nanoparticles are prepared by passing an inert gas to a hydrophilic organic solvent containing an oxygen-containing iron salt and a hydrophilic polymer. The organic solvent has a boiling point of 150° C. or higher. The solvent is heated to a temperature of 150° C.-500° C. to obtain hydrophilic magnetic nanoparticles. The hydrophilic magnetic nanoparticle has a water flux value of 8-40 l/m2h that decreases 10% or less after use as a draw solute.
A major problem in existing magnetic nanoparticle based draw solutions is the large energy consumption in the forward osmosis technology due to preparation of suitable hydrophilic magnetic nanoparticles.
There are several publications disclosing methods for preparation of hydrophilic MNPs for use as solutes based on thermal decomposition techniques involving elevated temperatures and high-boiling point organic solvents. Similarly, publications are available describing the use of co-precipitation methods by an alkali at elevated temperatures.
The drawbacks in both of these methods are the high energy consumption and utilization of organic solvents.
Therefore, there is a need for a method which overcomes these disadvantages to render forward osmosis a competitive and attractive technology for use in e.g. water purification.